1. Field of the Invention
The present invention relates to a switch circuit to be installed within a host device.
2. Description of the Related Art
USB (Universal Serial Bus) is today in widespread use as a standard for connecting a computer with peripheral devices. USB is widely incorporated in stationary equipment, such as PCs, and also in mobile devices, such as mobile phones, smartphones, digital cameras, portable music players, game devices, and IC recorders.
These mobile devices are provided not only with USB terminals (ports) but also with terminals (ports) for insertion of headphone terminals or earphone terminals (both hereinafter collectively referred to as “headphone terminal”) and those for insertion of hands-free microphones or recording microphones (hereinafter referred to as “microphone terminal”). To achieve downsizing or better design for such mobile devices, there have been proposals aiming for the use of a single common terminal (port) for the group of these terminals.
As a transistor for switching the analog audio signals passing through a headphone terminal or a microphone terminal, it is necessary that a transistor of low on-resistance of about several ohms be used. That is, the size of the transistor must be large. More specifically, the transistor must be of such design as to have a large gate width (GW) (i.e., a large diffusion layer).
On the other hand, it is known that the USB2.0 standard supports high-speed-mode transmission. In the high speed mode, large-volume data, such as video data, can be transmitted at high speed, namely, at a maximum bit rate of 480 Mbps. In order to switch high-frequency signals at such high speed, it is necessary that the load capacity at the common terminal for a group of terminals be as small as practicable. Large load capacities can dull rising edges and falling edges. That is, for the switching of high-frequency signals, the load capacity must be as small as practicable.
As described above, when a common terminal is used in the place of a USB terminal and a headphone terminal, a signal line is branched into a signal line for USB and a signal line for the headphone. And the high-frequency signals passing through the signal line for USB are more susceptible to distortion because of the capacity of the transistor switch inserted in the signal line for the headphone. The greater the number of signal lines for the transmission of analog audio signals to be branched off from the signal line for USB is, the greater this effect will be.